1. Field of the Invention
The present invention relates to an electromechanical transducer to be used for an ultrasound probe of an ultrasound diagnosis apparatus and the like, and to a method for manufacturing the same.
2. Description of the Related Art
An electromechanical transducer such as an ultrasound transducer conducts at least one of conversion from an electric signal to an ultrasound wave and conversion from the ultrasound to the electric signal, and it is used as a probe for ultrasound diagnosis for medical application, a probe for a non-destructive test and the like. In recent years, along with the development of a fine processing technology, a capacitance type electromechanical transducer (CMUT: Capasitive Micromachined Ultrasonic Transducer) produced by using the technology has been actively developed. An exemplary CMUT has a structure that an element substrate having a cell includes a substrate having a lower electrode (where the substrate occasionally serves as lower electrode as well), a vibrating membrane formed on the substrate so as to keep a fixed space between itself and the lower electrode, and an upper electrode, which element substrate being electrically connected with a driving circuit substrate. Furthermore, the CMUT has a structure in which the element substrate has a plurality of elements to which two or more cells are electrically connected, which element substrate being electrically connected to the driving circuit substrate (see Japanese Patent Application Laid-Open No. 2006-319712). A CMUT with high ability in transmitting or receiving the ultrasound by using a lightweight vibrating membrane, and having excellent broadband characteristics in any of a liquid and an air, can be easily obtained. Consequently, since the use of CMUT enables diagnosis with higher accuracy than that of a conventional medical diagnosis, and CMUT has received attention as a promising technology.
In operating CMUT, when transmitting the ultrasound, the CMUT applies a DC voltage and an AC voltage between the lower electrode and the upper electrode so that the voltages are superimposed. Thereby, the vibrating membrane vibrates and transmits the ultrasound. When receiving the ultrasound, the CMUT detects a signal from a change in the capacitance between the lower electrode and the upper electrode due to the change of a distance between both of the electrodes along with the deformation of the vibrating membrane, which occurs when the vibrating membrane has received the ultrasound. The methods for applying voltage to electrodes for driving the CMUT include: providing an electrode on the surface of the substrate of the CMUT and routing wires which are connected to the upper and lower electrodes; and guiding wires from the upper and lower electrodes to the rear surface of the substrate by using through wiring provided on the substrate, and thereby electrically connecting the electrodes to each other. In the former method, since it is necessary to route the wires on the surface of the substrate, an element can hardly be arranged in a portion occupied by the wire. Therefore, a fill factor, which is expressed by a ratio of the cells occupying in the elements having the same area, decreases. In addition, as for the space between the elements, since it is necessary to arrange the elements so as to be separated from each other by the area occupied by the wire, the elements can hardly be arranged at a high density. As a result, the performance of the CMUT is lowered. On the other hand, in the method of using the through wiring, normally, every element is insulated from each other in the substrate, and the through wiring is formed for the respective elements to electrically connect the electrodes. Thus manufactured CMUT is described in Japanese Patent Application Laid-Open No. 2007-215177 and Japanese Patent Application Laid-Open No. 2010-35134. Since there is need to route wires on the surface of the substrate in the CMUT formed by using the through wiring, the cell can be arranged in the portions occupied by the wires and the element can be arranged at a high density. Therefore, the CMUT having a high fill factor and a high arrangement density of the element can be produced, which leads to the enhancement of the performance.